Facile Synthesis of a Bi2WO6/BiO2−x Heterojunction for Efficient Photocatalytic Degradation of Ciprofloxacin under Visible Light Irradiation

Abstract

In this work, a Z-scheme Bi₂WO₆/BiO_2−x heterojunction was successfully prepared using a self-assembly strategy. Various characterization techniques demonstrated that the formation of the heterojunction not only accelerated the separation of photoinduced carriers but also weakened the recombination rate of photogenerated electron–hole pairs-. The Bi₂WO₆/BiO_2−x composites had a wider absorption edge than Bi₂WO₆ in the range of 200–800 nm, which improved the photocatalytic performance of ciprofloxacin (CIP) degradation under xenon lamps. As a result, the Z-scheme heterojunction Bi₂WO₆/BiO_2−x composite exhibited excellent photocatalytic activity. Catalyzed by the optimal 20% Bi₂WO₆/BiO_2−x (0.5 g/L), the removal rate of CIP (10.0 mg/L) was 91.8% within 2 h irradiated by visible light, which was 2.37 times that of the BiO_2−x catalyst. This work will provide a fresh perspective on the construction of visible-driven Z-scheme photocatalysts for wastewater treatment.

1. Introduction

Nowadays, antibiotics play an influential role in healthcare, agriculture and animal husbandry [1]. For example, ciprofloxacin (CIP), a second-generation quinolone antibacterial drug with broad-spectrum antibacterial and bactericidal activity, is commonly used to treat bacterial infections in humans and animals [2]. However, these drugs are not easily broken down in the living body; they remain in feces and eventually accumulate in rivers, pools and groundwater [3]. Through wastewater discharges and agricultural runoff, residual drugs can easily enter the aquatic environment, leading to the proliferation of drug-resistant bacteria that can lead to serious ecological risks [4]. It is imperative that antibiotics such as CIP should be removed from the ecosystem to ensure public and environmental health.

In recent years, photocatalysis has been regarded as an environmental purification technology that can effectively alleviate environmental problems due to its eco-friendly, safe and efficient characteristics [5,6]. Among the available photocatalyst materials, Bi₂WO₆ has attracted widespread attention owing to the low toxicity, favorable stability, and abundant oxygen defects [7]. Nonetheless, Bi₂WO₆ exhibits a high recombination rate of combined electron–hole pairs, which limits practical applications in environmental purification [8,9]. A series of strategies were attempted to alleviate the recombination rate of photogenerated electrons (e⁻) and holes (h⁺), including morphology control [10,11], doping [12], fabricating defective materials [13,14], and constructing heterojunctions [15,16]. It is prevalent to couple Bi₂WO₆ with the different functional materials to form heterostructures, which has been identified as the most useful approach. Among them, the construction of Z-scheme heterojunction has been proved to be an effective method to inhibit the recombination of electron–hole pairs [17]. Successful cases include Bi₂WO₆/BiPO₄ [18], Bi₂WO₆/CdS [19], Bi₂WO₆/CuBi₂O₄ [20], Bi₂WO₆/Ta₃N₅ [21], Bi₂WO₆/WO₃ [22], Bi₂WO₆/TiO₂ [23] and Bi₂WO₆/ZnO [24], and so forth.

A surprising finding in recent years is that all inorganic bismuth-based compounds have bright prospects for solar energy conversion and environmental remediation [25,26,27]. Among these, BiO_2−x was extensively reported to be a promising candidate for photocatalytic applications due to its relatively narrow band gap and plentiful oxygen vacancies [28]. Meanwhile, owing to the existence of mixed Bi³⁺ and Bi⁵⁺, BiO_2−x overcomes the drawbacks of inadequate light absorption range of a monovalent compound [29]. Bi₂WO₆ and BiO_2−x have received a lot of attention in the field of photocatalysis and wastewater treatment field due to their great potential for applications. However, few studies focus on the Bi₂WO₆/BiO_2−x-based heterojunction photocatalysis for CIP degradation. At the same time, the mechanism of the photocatalytic reactions in binary bismuth systems with oxygen defects is still unclear.

Here, a series of Z-scheme Bi₂WO₆/BiO_2−x heterostructures were fabricated using a facile electrostatic self-assembly process [30]. The photocatalytic performance of the—catalysts was verified by measuring its photodegradation performance of CIP. The application of a number of catalysts to the photocatalytic degradation of ciprofloxacin is shown in Table S3, demonstrating that synthesized heterostructures have a prominent effect on the photocatalytic degradation of ciprofloxacin. Electrochemical tests and photoluminescence (PL) spectra proved that the participation of Bi₂WO₆ could effectively inhibit the recombination of photogenerated carriers in the Bi₂WO₆/BiO_2−x heterojunction. In short, constructing Bi₂WO₆/BiO_2−x heterojunction restrains the recombination of photogenerated carriers, therefore optimizing broad-spectrum driven photocatalytic activity [31]. In addition, based on the photocatalyst characterization and experimental evidence, this work provides a comprehensive understanding of the constructed Z-scheme heterojunction for CIP degradation processes and paves a possible way for the development of extremely efficient photocatalysts for wastewater treatment.

2. Results and Discussions

2.1. Synthesis and Characterization of Catalysts

The crystalline structures of the parent BiO_2−x, Bi₂WO₆ and Bi₂WO₆/BiO_2−x composites were analyzed through their XRD patterns. As shown in Figure 1a, the characteristic peaks of BiO_2−x were located at 2θ = 28.21°, 32.69°, 46.92°, 55.64°, and 58.54°, corresponding to the (111), (200), (220), (311), and (222) crystal faces, respectively (JCPDS NO. 47-1057) [32], whereas the peaks at 2θ = 28.30°, 32.67°, 46.97°, 55.66°, and 58.33° were assigned to the (131), (060), (260), (191), and (262) planes of the Bi₂WO₆ crystal structure (JCPDS No. 39-0256) [33]. As shown in Table S1, we calculated the XRD data using the Scherrer formula, and found that the average grain size of the sample decreased with the increase in Bi₂WO₆ loading. The decrease of catalyst grain size will increase the specific surface area, which can further increase the contact area with the reactant and thus improve the photocatalytic performance. The increase in FWHM in Bi₂WO₆/BiO_2−x is attributed to lattice strain in the heterostructure, which may be due to the doping [17]. The Bi₂WO₆/BiO_2−x composites displayed similar structural features to BiO_2−x and Bi₂WO₆. It reveals that the crystal structure of the composite remains stable during synthesis, further confirming the heterostructure formation of Bi₂WO₆/BiO_2−x.

The FT-IR spectra of a series of Bi₂WO₆/BiO_2−x composites exhibited similar vibration modes to BiO_2−x, suggesting small amounts of Bi₂WO₆ combined with BiO_2−x did not affect the chemical structure of BiO_2−x (Figure 1b). In the spectrum of BiO_2−x, the absorption peaks at 519 cm⁻¹, 589 cm⁻¹and 954 cm⁻¹ belonged to the stretching vibrations of the Bi-O bond [33]. For Bi₂WO₆, the characteristic peaks at 589 cm⁻¹ and 687 cm⁻¹ could be ascribed to the stretching vibration of Bi-O and W-O [34]. The peak at 1389 cm⁻¹ was associated with the W-O-W bridging stretching of Bi₂WO₆ [35]. There were two signal peaks located at 1635 cm⁻¹ and 3421 cm⁻¹ that belong to the stretching and bending vibrations of the O-H functional groups resulting from the adsorption water, respectively [36]. All these characteristic peaks of BiO_2−x and Bi₂WO₆ existed in their composites, which proved that Bi₂WO₆/BiO_2−x heterojunction had been successfully formed.

Furthermore, Figure 1c shows the Raman spectroscopy of Bi₂WO₆. For the pure Bi₂WO₆ sample, the peaks at 158 cm⁻¹, 305 cm⁻¹ and 825 cm⁻¹ were assigned to the Bi-O, the bending vibrations mode of BiO₆ polyhedron and the symmetric stretching vibration of O-W-O [37,38]. Figure 1d shows the Raman spectra of BiO_2−x and a series of Bi₂WO₆/BiO_2−x composites ranging from 100 cm⁻¹–1200 cm⁻¹. The peaks at approximately 135 cm⁻¹, 480 cm⁻¹ and 615 cm⁻¹ could be ascribed to the Bi-O stretches of BiO_2−x [39]. With the increase in content of Bi₂WO₆ in the composites, the intensity of the peak at 305 cm⁻¹ also increased, indicating that the Bi₂WO₆/BiO_2−x heterojunction was successfully synthesized.

To acquire the porous characteristics of the obtained materials, we proceeded with N₂ adsorption–desorption experiments for BiO_2−x, Bi₂WO₆ and 20% Bi₂WO₆/BiO_2−x. Figure 1e and Table S2 displayed the N₂ adsorption–desorption isotherms of the samples; the specific surface area of BiO_2−x, Bi₂WO₆ and 20% Bi₂WO₆/BiO_2−x were 5.1639 m² g⁻¹, 101.5031 m² g⁻¹ and 10.4902 m² g⁻¹. According to the IUPAC classification, all of the materials exhibited type IV isotherms featured with type H3 hysteresis loops, suggesting these samples contain abundant mesoporous [40,41]. The porous features can be further found in their pore size distribution curves shown in Figure 1f; the pore sizes of Bi₂WO₆, BiO_2−x and 20% Bi₂WO₆/BiO_2−x were dispersed between 8.7 and 53.7 nm. Obviously, the specific surface area of 20% Bi₂WO₆/BiO_2−x was approximately twice that of pure BiO_2−x, indicating that the composites had more active sites and were beneficial to enhancing photocatalytic performance.

Figure S1a shows the typical nanosheet morphology of the BiO_2−x nanostructure. Bi₂WO₆ is formed by the accumulation of a large number of nanosheets (Figure S1b). Figure S1c and Figure 2a show the SEM and TEM images of 20% Bi₂WO₆/BiO_2−x, which prove that Bi₂WO₆ nanosheets are loaded on BiO_2−x nanosheets successfully. The HRTEM images (Figure 2b and Figure S2b) show the lattice stripes of 0.315 nm, corresponding to the (111) crystal plane of BiO_2−x nanosheets. Moreover, Figure 2b and Figure S2d show that the calculated d space values 0.375 nm and 0.226 nm are attributed to the (111) plane and (042) plane of Bi₂WO₆. EDS elemental analysis (Figure S3), and the elemental mapping images (Figure 2d–f) of 20% Bi₂WO₆/BiO_2−x reveal homogeneous distribution of Bi, O, and W elements in the sample, which is highly in accordance with the results of TEM images and indicates the formation of Bi₂WO₆/BiO_2−x heterojunction.

X-ray photoelectron spectroscopy (XPS) technology was performed to elucidate the surface element composition and valence state of the prepared catalysts and the interaction between BiO_2−x and Bi₂WO₆. As shown in Figure 3a, in addition to the characteristic peaks of Bi and O, there are also peaks of W in the 20% Bi₂WO₆/BiO_2−x heterostructure. Figure 3b–d shows the XPS high-resolution spectra of O, Bi and W XPS, respectively. For Bi₂WO₆/BiO_2−x hybrid, the deconvolution of the high-resolution spectrum of O 1s (Figure 3b) gives three peaks at 529.5 eV, 530.4 eV and 531.8 eV, assignable to lattice oxygen [42], surface hydroxyl group [37] and oxygen vacancies [43] in the as-prepared samples, respectively. The Bi 4f_7/2 and 4f_5/2 peaks are situated at 158.3 eV and 163.6 eV, 159.1 eV and 164.5 eV for BiO_2−x and Bi₂WO₆, respectively [39]. Furthermore, the peak positions of Bi 4f for 20% Bi₂WO₆/BiO_2−x were positively shifted relative to BiO_2−x, which were negatively shifted relative to Bi₂WO₆ (Figure 3c). The W 4f_7/2 and W 4f_5/2 peaks of 20% Bi₂WO₆/BiO_2−x are centered at 35 eV and 37.2 eV [44], which are 0.4 eV lower than those of the Bi₂WO₆ sample (35.4 eV of W 4f_7/2 and 37.6 eV of W 4f_5/2) (Figure 3d). This phenomenon reveals the successful synthesis of Bi₂WO₆/BiO_2−x heterojunction.

2.2. Optical and Photoelectrochemical Property Analysis

Figure 4a displays the UV–Vis DRS spectra of the prepared photocatalysts. The absorption edge of 20% Bi₂WO₆/BiO_2−x displayed an obvious red shift compared with that of Bi₂WO₆, suggesting that composites could make better use of visible light. The light absorption edge of Bi₂WO₆ had a band gap of 2.54 eV, while BiO_2−x had a narrower band gap of 1.34 eV (Figure 4b). The positive slope of the Mott–Schottky curves in Figure S4a,b indicates that both Bi₂WO₆ and BiO_2−x are n-type semiconductors and the flat band positions of Bi₂WO₆ and BiO_2−x are −0.289 and −0.436 V (relative to Ag/AgCl). Furthermore, according to the formulas of E_NHE = E_(Ag/AgCl) + 0.197 and E_CB = E_{flat band} − 0.1 (n-type semiconductor), the E_CB values of Bi₂WO₆ and BiO_2−x are calculated to–0.192and −0.339V vs. NHE, respectively. Additionally, based on the formula E_VB = E_CB + E_g, the valence band positions (E_VB) of Bi₂WO₆ and BiO_2−x are calculated to −2.348 and −1.001eV vs. NHE, respectively. As shown in Figure 4c, such band combinations of Bi₂WO₆ and BiO_2−x were conducive to the formation of the heterojunction. Furthermore, the photoelectrochemical measurement was also evaluated to investigate the charge separation and transfer efficiency of samples. From Figure 4d, with light switching on and off, the photocurrent densities of the 20% Bi₂WO₆/BiO_2−x composite demonstrated the highest photocurrent density over other samples, and also presented the lowest resistance ability, indicating enhanced separation of photogenerated carriers in the composites. According to the electrochemical impedance spectroscopy (EIS) Nyquist plots (Figure 4e), compared with pure Bi₂WO₆ and BiO_2−x, the formation of Bi₂WO₆/BiO_2−x heterojunction could significantly reduce the semi-arc, indicating that the carrier recombination rate of the composite was lowest. In order to further detect the photogenerated electrons and holes recombination, PL emission spectra is commonly employed. As depicted in Figure 4f, the PL intensity of Bi₂WO₆ significantly decreased when it hybridized with BiO_2−x, indicating that the 20% Bi₂WO₆/BiO_2−x samples obtained the superior charge separation ability. In a word, the separation efficiency of photogenerated carriers in the 20% Bi₂WO₆/BiO_2−x samples could be greatly enhanced, which could significantly contribute to improving its photocatalytic activity.

2.3. Photocatalytic Performance of Catalysts

The photocatalytic performances of BiO_2−x, Bi₂WO₆ and a series of Bi₂WO₆/BiO_2−x composites were explored through CIP degradation under visible light illumination. As indicated in Figure S5, no remarkable CIP decomposition was found without any catalyst under the illumination of visible light, which indicates that the presence of catalysts is necessary for the degradation of CIP. As Figure 5a,b showed, the degradation percentage of 91.8% was achieved by 20% Bi₂WO₆/BiO_2−x after 120 min of degradation process and its corresponding apparent kinetic constant was 0.02240 min⁻¹, which was higher than that of BiO_2−x by 2.37 times. Thus, the synergistic effect can be achieved using the combination of Bi₂WO₆ and BiO_2−x via ultrasonication, which may be ascribed to the fact that the formation of Bi₂WO₆/BiO_2−x heterojunctions can enhance visible light absorption efficiency, accelerate the transfer rate of photo-generated electron–hole pairs and reinforce the oxidation ability for CIP decomposition.

To evaluate the stability of 20% Bi₂WO₆/BiO_2−x, cyclic experiments for the photodegradation of CIP were performed in Figure 5c. 20% Bi₂WO₆/BiO_2−x could retain its activity well for four catalytic cycles, indicating the great stability of 20% Bi₂WO₆/BiO_2−x. Moreover, the SEM image of the used 20% Bi₂WO₆/BiO_2−x was almost the same as the fresh one (Figure S1c,d). Furthermore, in the XRD pattern before and after the photocatalytic degradation of CIP of 20% Bi₂WO₆/BiO_2−x, it was found that the position of the characteristic peak did not shift, further demonstrating the structural stability during the photocatalysis process (Figure S7). The FTIR of CIP (Figure S6a) showed peaks corresponding to the asymmetric -CH₃ stretching vibrations at 2924.06 cm⁻¹, ring vibrations at 1036.7 cm⁻¹, amidogen ν_N-H vibrations at 3600 to 2700 cm⁻¹ and the -C-N band finger prints of the azo nature of dye at 1119.1 cm⁻¹ [45]. Most importantly, these characteristic peaks of CIP almost disappeared in the presence of 20% Bi₂WO₆/BiO_2−x (Figure S6b–d), suggesting the degradation of CIP. These results indicated that the as-prepared 20% Bi₂WO₆/BiO_2−x maintained stable photocatalytic activity during long time photo-degradation reactions.

As shown in Figure 5d, the influence of-pH on the photo-degradation of CIP in suspensions of 20% Bi₂WO₆/BiO_2−x was investigated in the pH range of 3.0–11.0. The dosage of 20% Bi₂WO₆/BiO_2−x and CIP concentration were 0.5 g/L and 10 mg/L, respectively. It can be seen that when the alkalinity of the solution was increased, the degradation efficiency of CIP was inhibited. Although the CIP removal decreased at pH 11.0, about 53.6% of CIP was still removed, proving that 20% Bi₂WO₆/BiO_2−x system could be applied over a wide pH range. According to Figure 5e, Na⁺, K⁺, Ca²⁺ and Mg²⁺ had little effect on CIP removal. In addition, the inorganic anion NO₃⁻ had little effect on the degradation of CIP, while the CO₃²⁻ had a significant inhibitory effect on CIP degradation (Figure 5f). Conclusively, the 20% Bi₂WO₆/BiO_2−x system exhibited excellent photo-degradation of organic pollutants.

2.4. Possible Degradation Pathways

In order to explore the photocatalytic degradation mechanism and possible degradation pathways of CIP, the intermediate products in the degradation of CIP were estimated using HPLC-MS. There were two possible photocatalytic degradation pathways of CIP, as shown in Figure 6. Pathway 1, CIP (m/z = 332), was attacked by the active species and produced the intermediates of products A (m/z = 362). Then, A (m/z = 362) suffered decarboxylation to form B (m/z = 334) and was further defluorinated to form E (m/z = 344) [46]. Subsequently, B (m/z = 334) lost both -CO and -C₂H₅N to form C (m/z = 263), which defluorinated to D (m/z = 245), respectively. F (m/z = 316) lost -CO and was transformed to G (m/z = 288), which subsequently lost -C₂H₅N to form D (m/z = 245) and formed H (m/z = 190) and I (m/z = 104) through further oxidation. Then, I (m/z = 104) degraded into small molecular pollutants. Pathway 2, CIP (m/z = 332), suffered decarboxylation to form J (m/z = 288). Afterward, the piperazine and adjacent C=C in quinolone structure of intermediate J (m/z = 288) could be further oxidized and allowed the formation of intermediate K (m/z = 243). Then, the closing of a five-membered ring on intermediate K (m/z = 243) induced to the formation of intermediate L (m/z = 181) [47]. The side groups of the middle L (m/z = 181) lost with the cracking of the five-membered ring to form the middle M (m/z = 171). Finally, these products further oxidized and mineralized into CO₂ and H₂O.

2.5. Mechanism Discussion

In order to clarify the main reactive radical species on the degradation of CIP, we carried out scavenging experiments using tertiary butanol (TBA), Na₂C₂O₄, p-benzoquinone (PBQ) and furfural alcohol (FFA) for quenching hydroxyl radicals (•OH), photo-generated holes (h⁺), superoxide radicals (•O₂⁻) and singlet oxygen (¹O₂), respectively [48]. As Figure 7a,b shows, the decomposition efficiency of CIP almost remained when TBA was added to the suspension, implying that less •OH was produced during CIP degradation. In contrast, a sharp decrease in decomposition efficiency was observed when Na₂C₂O₄, PBQ and FFA were added in system; the degradation efficiency of CIP decreased to 39.4%, 45.7% and 42.5%, and the corresponding reaction rate constant decreased to 0.00557 min⁻¹ and 0.00580 min⁻¹and 0.00590 min⁻¹. These results indicated that h⁺ played a major role in CIP degradation, •O₂⁻ and ¹O₂ were the moderate reactive species in the degradation of CIP over the 20% Bi₂WO₆/BiO_2−x composite. To verify this conclusion, we conducted ESR experiments in Figure 7c–f. Upon visible light illumination, the corresponding characteristic peaks intensity of DMPO-•O₂⁻, TEMP-¹O₂ and DMPO-•OH in the ESR spectra slightly increased, while the characteristic peaks’ intensity of TEMPO-h⁺ decreased significantly after illumination. As illustrated in Figure 7e, the three peaks’ intensity of TEMPO-h⁺ would get weaker, demonstrating the existence of photogenerated h⁺ [6]. Conclusively, the existence of •O₂⁻, ¹O₂, •OH and h⁺ in CIP photodegradation process was confirmed.

According to the aforementioned results, the possible CIP photodegradation mechanism was presented in Scheme 1. Under visible light irradiation, both Bi₂WO₆ and BiO_2−x could be photoexcited and produced electron–hole pairs, and the photo-induced electrons migrated from the valence band to the conduction band. Since the E_CB of Bi₂WO₆ was more -positive than the O₂/•O₂⁻ potential (−0.33 V vs. NHE), •O₂⁻ could not be generated in the CB. Therefore, the type II heterojunction did not conform to the structure of this Bi₂WO₆/BiO_2−x composites, that a Z-scheme charge transfer way was proposed for this degradation process. The photogenerated electrons in the CB of Bi₂WO₆ could migrate to the VB of BiO_2−x and combine with h⁺ by visible light illuminating. In the VB of Bi₂WO₆ and the CB of BiO_2−x, •OH and •O₂⁻ could be produced, respectively. Moreover, most of the •O₂⁻ further reacted to generate ¹O₂. These active species gradually converted CIP into small molecules that were easily broken down. Combined with ESR and radical quenching reactions, the overall electron transition and CIP degradation reaction in Bi₂WO₆/BiO_2−x heterostructures were proposed as follows (Equations (1)–(7)) [49,50,51]:

(1)${\mathrm{Bi}}_2{\mathrm{WO}}_6/{\mathrm{BiO}}_{2-\mathrm{x}}\stackrel{\mathrm{h}\mathrm{\nu }}{\to }{\mathrm{h}}^{+}+{\mathrm{e}}^{-}$

O₂ + e⁻ → •O₂⁻(2)

H₂O ↔ H⁺ + OH⁻(3)

•O₂ ⁻ + 2H⁺ → 2•OH(4)

h⁺ + OH⁻ → •OH(5)

h⁺ + •O₂ ^{→ 1}O₂(6)

h^+/•O₂^−/1O_2/•OH + CIP → CO₂ + H₂O(7)

3. Experimental

3.1. Materials

NaBiO₃⋅2H₂O, Bi(NO₃)₃⋅5H₂O, Na₂WO₄⋅2H₂O, tertiary butanol, Na₂C₂O₄ and p-benzoquinone were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Ethylene glycol and furfural alcohol were purchased from Aladdin Reagents Co., Ltd. (Shanghai, China). Ciprofloxacin hydrochloride monohydrate was used as CIP raw material, which was produced by the Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).

3.2. Preparation of BiO_2−x

BiO_2−x was prepared using a hydrothermal method. Briefly, 1.2 g NaOH and 1.4 g NaBiO₃⋅2H₂O were incorporated into 30 mL of ultrapure water via magnetic stirring. After stirring for 30 min, the suspension was transferred into a 50 mL autoclave, and heated at 180 °C for 18 h. The mixture was naturally cooled to room temperature and collected via centrifugation, washed with ultrapure water and dried at 80 °C for 4 h under vacuum.

3.3. Preparation of Bi₂WO₆

Firstly, 0.3881g Bi(NO₃)₃⋅5H₂O and 0.1319 g Na₂WO₄⋅2H₂O were mixed together in 40 mL of ethylene glycol and stirred for 30 min to obtain the homogeneous solution. It was transferred into a 100 mL Teflon-lined autoclave and treated at 160 °C for 15 h. After cooling at 25 °C, the product was centrifuged and rinsed several times with ethanol and ultrapure water to remove residual inorganic ions. Finally, Bi₂WO₆ powders were oven-dried at 70 °C for 12 h.

3.4. Preparation of Bi₂WO₆/BiO_2−x

The Bi₂WO₆/BiO_2−x composite was prepared via an electrostatic self-assembly method illustrated in Scheme 2. A total of 0.10 g of BiO_2−x and different amounts of Bi₂WO₆ (0.005, 0.01, 0.015, 0.02 and 0.025 g) were added to 30 mL ethanol with ultrasonics for 4 h, and dried at 60 °C for 4 h under vacuum.

4. Conclusions

In summary, we have successfully prepared a Z-scheme Bi₂WO₆/BiO_2−x heterostructures using a simple method with excellent photocatalytic CIP degradation performance under visible light irradiation. By constructing a Z-scheme heterostructure, the charge recombination rate was significantly reduced in the photocatalytic degradation of ciprofloxacin. The Z-scheme charge transfer mechanism in Bi₂WO₆/BiO_2−x heterojunction as investigated via XPS, photoelectrochemical measurements and ESR experiment greatly enhances the separation of photogenerated carriers to expedite CIP photodegradation. With the increasing Bi₂WO₆ content, the average grain size of the samples decreases, and the surface area of the materials in contact with the pollutant increases, which improves the photocatalytic performance. The 20% Bi₂WO₆/BiO_2−x samples were constructed to alleviate the problem of the high recombination rate of photogenerated e⁻-h⁺, and the CIP degradation rate reaches 91.8% under the present conditions. The 20% Bi₂WO₆/BiO_2−x heterostructures still had good stability and photocatalytic activity after four cycles, showing high potential in practical applications. Critically, this study demonstrates the successful application of CIP degradation and provides valuable insights regarding the development of the construction of heterojunctions to further improve their photocatalytic degradation efficiency.

Author Contributions

Conceptualization, H.Z. and J.L.; methodology, software, validation, formal analysis, investigation, resources, H.Z.; data curation, writing—original draft preparation, H.Z. and Z.F.; writing—review and editing, visualization, Z.F. and Q.C. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Data Availability Statements are available in section “MDPI Research Data Policies” at https://www.mdpi.com/ethics (accessed on 1 February 2023).

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

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Figures and Schemes

Figure 1. (a) XRD patterns and (b) FT-IR spectra of BiO2−x, Bi2WO6 and Bi2WO6/BiO2−x composites. Raman spectra of (c) Bi2WO6, (d) BiO2−x and Bi2WO6/BiO2−x composites. (e) N2 adsorption–desorption isotherms and (f) BJH pore diameter distribution of BiO2−x, Bi2WO6 and 20% Bi2WO6/BiO2−x.

Figure 2. (a) TEM image; (b) HRTEM image; (c) HAADF image and elemental mapping of 20% Bi2WO6/BiO2−x, (d) Bi; (e) O and (f) W elements distribution.

Figure 3. XPS spectrum of BiO2−x, Bi2WO6 and 20% Bi2WO6/BiO2−x: (a) survey, (b) Bi 4f, (c) O 1s and (d) W 4f.

Figure 4. (a) DRS spectra of of BiO2−x, Bi2WO6 and 20% Bi2WO6/BiO2−x; (b) Tauc plots, (c) Band structures of BiO2−x and Bi2WO6; (d) Transient photocurrent response, (e) electrochemical impedance spectra and (f) photoluminescence fluorescence spectra of BiO2−x, Bi2WO6 and 20% Bi2WO6/BiO2−x.

Figure 5. (a) Photocatalytic degradation of CIP using various photocatalysts irradiated with visible light and (b) their corresponding kinetic curves, (c) The cyclic experiment of the 20% Bi2WO6/BiO2−x system for degradation of CIP. The effects of (d) initial solution pH, (e) inorganic cations and (f) anions in 20% Bi2WO6/BiO2−x system for degradation of CIP under visible light irradiation. (Condition: sample dosage = 0.5 g/L, CIP concentration: 10 mg/L).

Figure 6. Proposed degradation pathways of CIP by the Bi2WO6/BiO2−x nanocomposite.

Figure 7. (a) Photodegradation of CIP in a 20% Bi2WO6/BiO2−x system with different quenchers and (b) corresponding kinetic plots. ESR spectra of various catalysts for (c) DMPO-•O2− adduct; (d) TEMP-1O2 adduct; (e) TEMPO-h+ adduct and (f) DMPO-•OH adduct.

Scheme 1. The proposed photocatalytic mechanism for photodegradation of CIP over the Bi2WO6/BiO2−x composites.

Scheme 2. Schematic illustration of the preparation of Bi2WO6/BiO2−x composites.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal13030469/s1, Table S1: Crystallite size of the prepared materials [17]; Table S2: Porous parameters of BiO_2−x, Bi₂WO₆ and 20% Bi₂WO₆/BiO_2−x [15]; Figure S1: SEM image of (a) BiO_2−x; (b) Bi₂WO₆; 20% Bi₂WO₆/BiO_2−x (c) before and (d) after use [5]; Figure S2:TEM images of (a) BiO_2−x; (c) Bi₂WO₆ and HRTEM images of (b) BiO_2−x; (d) Bi₂WO₆ [47]; Figure S3: EDX spectrum of 20% Bi₂WO₆/BiO_2−x [48]; Figure S4: The Mott-Schottky plots of (a) Bi₂WO₆ and (b) BiO_2−x at different frequencies (500 Hz, 1000 Hz and 1500 Hz) [41]; Figure S5: Photocatalytic degradation of CIP without any catalyst for comparison tests. (condition: sample dosage = 0.5 g/L, CIP concentration: 10 mg/L) [1]; Figure S6: FT-IR spectra of (a) CIP, (b) 20% Bi₂WO₆/BiO_2−x (c) 20% Bi₂WO₆/BiO_2−x after mixing with CIP in the dark for 30 min, and (d) 20% Bi₂WO₆/BiO_2−x after mixing with CIP in the dark for 30 min and then under visible light irradiation for 2 h [52]; Figure S7: XRD patterns of BiO_2−x, Bi₂WO₆, fresh and used Bi₂WO₆/BiO_2−x composites [53]; Figure S8: Bode plots of BiO2-x, Bi₂WO₆ and 20% Bi₂WO₆/BiO_2−x [54]; Figure S9: Photocatalytic degradation of CIP with prolonged light time for comparison test. (condition: sample dosage = 0.5 g/L, CIP concentration: 10 mg/L) [55]; Table S3. The catalytic performance comparison of recently reported catalysts for CIP degradation based on reduction percentages and reaction time.

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